This invention relates generally to a beam index signal processing system and, particularly, to a low cost, wide-band beam index video-index signal system.
Interest in so-called beam index color television cathode ray tubes (CRTs) has accompanied the rise in popularity of computer video terminals, particularly video terminals requiring high resolution color displays. In that environment, the high resolution capability of the beam index CRT affords obvious advantages over conventional color CRTs, even those of the high resolution type. Further, the information on the computer video terminal is generally in a form that is adaptable to digital control and readout. Usually the CRT display is driven by a character generator, under control of a microprocessor, for displaying a number of characters or pixels of information and the sharpness and resolution of the video display is of great importance. As will be shown, in a wide-band index tube system, the position of the single beam is at all times known with precision and thus the beam index CRT may advantageously be used as a high resolution color monitor.
Beam index systems may be generally categorized as narrow-band and wide-band. The more common narrow-band system operates on the basis that the beam scanning frequency is fixed throughout a scan line with minor adjustments in phase being made during the scanning of the line. A narrow-band system has the advantage of noise immunity and can therefore operate with very low index strip excitation. A disadvantage is that all but very minor variations in space between index strips are generally not compensatable and can result in color impurity. A wide-band index system operates on the basis that the scanning is done on a strip-by-strip basis which automatically compensates for variations in periodicity of the index strips. Examples of a wide-band index system are U.S. Pat. Nos. 4,408,223 and 4,468,690. The wide-band system affords very close control over the beam and enables improved efficiency because of the precision with which the beam position is known. A disadvantage is that the wide frequency band renders the system susceptible to noise and the excitation level of the index strip must be much greater to produce an acceptable signal-to-noise ratio. The high index strip excitation adversely impacts the no video "blackness" of the tube and the contrast of the display. As described in the above patents, techniques are available for preserving these characteristics by turning off the beam current between the index strips, in the absence of video information.
A wide band index signal system must therefore be designed to operate with the relatively high beam current required to reliably "find" the index strips (the index video signal) and to drive the CRT sufficiently to produce a bright video display. Inevitable compromises occur since fast switching transistors of high current capacity are very expensive. In the prior art patents, mixing of the video and the index video signals is done before the video amplifier which imposes the requirements of a transistor video amplifier that is both powerful and linear. There requirements significantly add to the system cost. With the invention, the digital video and index video signals are supplied to a video-index signal mixer that operates primarily as a switch with the result that relatively low power, low cost "off-the-shelf" switch transistors may be used. The resultant circuit is not only cost effective, it exhibits improved operating characteristics.